If stenosis has occurred in a vessel of a living body, such as blood vessel, a balloon forming portion provided in the vicinity of the distal end of a balloon catheter is inserted into this stenosis portion. This balloon forming portion is dilated to expand the stenosis portion of the blood vessel to improve the blood flow. This operation, known as percutaneous angioplasty (PTA), has so far been in widespread use.
However, after application of PTA to the site of occurrence of stenosis in a blood vessel, acute occlusion, attributable to the dissection of the intima, or re-narrowing of the same site as that where narrowing in the blood vessel (stenosis) has occurred, that is, re-stenosis, tends to be produced in a well-known manner at a high probability.
For preventing such acute occlusion or re-stenosis, the technique of implanting a tubular stent at the target site where PTA has been applied, has so far been used. A stent, used for the purpose, is implanted into the blood vessel, as it is contracted in diameter, and subsequently enlarged in diameter so as to be implanted in the blood vessel to support the blood vessel wall from its inside.
For a stent implanted in the blood vessel, a stent made of metal has so far been used. The metal stent is classified into a balloon expanding stent and a self-expanding stent.
The balloon expanding stent is inserted into a targeted site for implantation in the blood vessel, in a state contracted in diameter, and subsequently enlarged in diameter with expansion of the balloon. Among the stents of this type, there are a stent comprised of a small-diameter tube of stainless steel provided with numerous incisions formed by e.g. a laser cutter to permit the tube to be enlarged in diameter, and a stent formed by braiding a fine metal filament into a tube, as disclosed in U.S. Pat. No. 4,950,227.
The self-expanded stent is contracted in diameter under application of an external pressure and inserted in this contracted state in the target site for implantation in the blood vessel. After removal of the external pressure, the stent is self-expanded in diameter to support the blood vessel from its inner wall surface. As typical of this type of the self-expanded stent, there is known such a one obtained by spirally winding a fine metal wire to form a tube, as disclosed in the Japanese Laid-Open Patent Publication Hei-2-68052.
For implanting the above-described stent for a vessel in the target site in the blood vessel of a living body, a stent delivery system is used. The stent delivery system is of variable configurations, depending on the type of the stent to be delivered, that is, on whether the stent delivered is the balloon expanding stent or the self-expanded stent.
The stent delivery system for delivering the balloon expanding stent within the blood vessel includes a catheter inserted into the blood vessel, and a balloon is provided, as it is contracted in diameter, to the distal end of the catheter. On this balloon is mounted a stent as it is contracted in diameter. The stent, mounted on the balloon, is pressed from its outer peripheral side and retained against detachment from the balloon. The stent, thus mounted on the balloon, is delivered as far as the targeted site for implantation in the blood vessel, along with the balloon, by progressively inserting the catheter into the blood vessel. The stent, thus delivered to the target site for implantation in the blood vessel, is expanded in diameter on plastic deformation caused by balloon expansion to support the blood vessel wall from its inner side.
For the stent delivery system, used for implanting the balloon expanding stent in the blood vessel, it is basically only sufficient to include a means for mounting a stent, contracted in diameter, on the balloon provided to the catheter.
As the stent delivery system for delivery of the balloon expanding stent, there has been proposed such a system including a sheath covering up the stent mounted on the balloon. The sheath used is provided for preventing the stent, mounted on the balloon, from becoming detached from the balloon.
On the other hand, the stent delivery system for delivery of the balloon expanding stent into the blood vessel is constructed so that a catheter mounting a stent contracted in diameter is inserted into a protective sheath. The stent, mounted in the state contracted in diameter in the catheter, is covered up by the protective sheath and thereby maintained in the state contracted in diameter. For implanting the stent in the target site for implantation, using the above-described stent delivery system, the catheter, mounting the stent, is inserted up to the target site for implantation in the blood vessel, along with the protective sheath. At this time, the catheter is fixed and only the protective sheath is retreated in the blood vessel, whereby the stent, mounted to the distal end of the catheter, is freed from the sheath. The stent, thus freed from the protective sheath, is self-expanded by elasticity proper to the stent itself, and is dilated in diameter to a size capable of providing a support for the inner wall of the blood vessel.
The stent delivery system, used for implanting the self-expanding stent in the blood vessel, includes a catheter on which is mounted a stent, contracted in diameter, and a protective sheath in which is housed the catheter, the stent has been mounted to, there being no necessity to provide a balloon for expanding the stent.
Currently, there has not been established a method for treatment for an instance where re-stenosis has occurred on a site where angioplasty has been applied and a metal stent has been implanted.
Moreover, if metal, inherently a foreign substance for the living body, is caused to remain for a prolonged time in the living body, there is a cause that the blood vessel may thereby be affected, such as by excessive intimal hyperplasia occurring in the stent implant portion.
With a view to obviating the problems inherent in the conventional metal stent, the present Assignee has already proposed a stent formed using a biodegradable polymer (see U.S. Pat. Nos. 6,045,568, 2,842,943 and WO00/13737).
The stent formed of the biodegradable polymer may be absorbed in the tissue of the blood vessel after a preset time, has passed after it is implanted in the blood vessel, for example, after lapse of 6 to 12 months, such that the function of providing a support for the blood vessel from the inner side thereof is no longer needed. Since the stent of this type may be absorbed in vivo, it becomes possible to suppress adverse effects which might be produced as a result of the stent, as a foreign material for the living body, being left over for a prolonged time.
In particular, the present Assignee has already proposed a stent for a vessel, formed by braiding a yarn of a biodegradable polymer into a tube (U.S. Pat. No. 6,045,568), a stent for a vessel prepared by forming a yarn of a biodegradable polymer in a non-woven non-braided state (U.S. Pat. No. 2,842,943) and a stent for a vessel prepared by bending a yarn of biodegradable polymer in a zigzag design to form concatenated vee shapes, and by winding the resulting zig-zag shaped yarn into a tube, with the stent for a vessel being expanded or contracted in diameter with vee shaped portions of the yarn as portions subjected to displacements (WO00/13737). These stents were actually implanted in living bodies.
The stent formed of the biodegradable polymer is formed into a tube and subsequently heat-set, by way of heat treatment, for shape retention to a desired outer diameter. This heat-setting is carried out at a temperature not lower than the glass transition temperature and not higher than the melting point of the biodegradable polymer making up the stent. The stent which is to be implanted in the blood vessel, and which has its shape retained to a desired outer diameter, is contracted in diameter for insertion into the blood vessel. This contraction of the stent is carried out under application of an external pressure with or without heat setting. The heat-setting here is carried out at a temperature lower than the temperature for heat setting carried out for retention of the expanded state.
The stent made of the biodegradable polymer is expanded by a balloon expansion method employing a balloon. This method is carried out for promptly expanding the stent, inserted in a state contracted in diameter as far as the site for implantation in the blood vessel, to a size capable of reliably supporting the inner wall of the blood vessel.
Meanwhile, the stent, formed using the biodegradable polymer, may be warmed and thereby given the self-expanding properties, that is, the properties of shape memory. When mounted on the catheter and inserted in this state into the blood vessel of the living body, the stent, formed of the biodegradable polymer, is self-expanded, as it is warmed by body temperature of the living body. Since the stent has the self-expanding properties, it is tightly contacted with the inner wall of the blood vessel to maintain the force of dilating the blood vessel from its inside, and hence is able to distend the blood vessel from its inner wall over a preset time period until the time of biodegradation.
Thus, the stent formed of the biodegradable polymer has the self-expanding properties, even though it necessitates expansion by the balloon. For inserting this sort of the stent into the blood vessel of the living body for implantation therein, there is needed, along with the balloon for expanding the stent, an expansion inhibiting member for inhibiting self-expansion of the stent which is otherwise caused when the stent is warmed up by body temperature on insertion thereof into the blood vessel. That is, for preventing the occurrence of an accident in which the stent contracted in diameter is self-expanded on being inserted into the blood vessel and is disengaged from the balloon, it becomes necessary to provide a protective sheath to control the self-expansion of the stent mounted to the balloon.
There is also the possibility that the stent of the biodegradable polymer, exhibiting the self-expanding properties, is jumped up from the protective sheath by its force of expansion and becomes disengaged from the catheter, when the stent, delivered to the targeted site for implantation in the blood vessel of the living body, is subsequently gradually freed of the support from the protective sheath, such that a given portion of the stent is protruded from the protective sheath. The result is that not only the stent cannot be implanted in the targeted site for implantation in the blood vessel but also the stent becomes unable to be expanded by the balloon.